Answers for Authors: How do I conquer another planet?

How do I conquer another planet?

I have assumed that the asker is researching for a book they are writing. 

Although... they didn't specify that they were. 

So my back up assumption is that they don't have the means to carry any of the following out.

If I'm wrong... er. Sorry everyone?

Anyway: The only inhabited planet we have to use as a model for conquest is Earth, so let's approach this from the the perspective of an extraterrestrial looking to invade.
I'm the alien. I'm considering investing in a fleet of massive space battlecruisers, possibly even a Death Star or two. Terrifying bioweapons might be on my mind.

Mwahahahahahah!

But... that's not what I actually need.

Why? Well, what will make the governments of Earth capitulate to my evil alien overlordship? And can I find a  method doesn't involve breaking my budget, because I'm fiscally responsible as well a pure tentacled evil? 

In fact... yes. There is a very simple method, one which mother nature herself has tried and tested. In fact it's so easy to do  the humans might just figure it out for themselves and blow their own planet up with it: 

I can just drop rocks on them.

'It can't be that simple' you're thinking? Well have a look at this feasibility study, prompted by NASA and being carried out by Made In Space Inc: They're looking at sending 3D printers to asteroids and turning the asteroid material into crude mechanical parts. Slowly, they could convert the asteroid into a huge (if basic) unmanned spacecraft. It could be very simple, and literally be powered by large springs - all because it doesn't need to climb any steep gravity wells like a ship leaving Earth does. 
Once converted it would gently steer around the solar system under commands from Earth*. Here's their concept design:

It would mean some major advances in 3D printing, but nothing unbelievable - Made In Space have already pushed the 3D space envelope, launching a 3D printer to the International Space Station and making tools with it there.

So far, so 'aint-it-cool'. But, while throwing 3D printing into the mix is new, there have been a lot of (fairly realistic) ways to change an asteroid's or comet's course already studied. To name just a few you could:

• Pull it with gravity tractor
• Land a spaceship on it and push it with an engine
• Set nukes off near its surface 

And these aren't just theories: NASA is, right now, in the process of designing and building a space mission that will  park an asteroid in an orbit around the Moon. Astronauts will explore it, and practise for missions to the moons of Mars.

Because it's pretty: Mars.

I'd say a steerable asteroid looks pretty possible with merely human technology.

That's great for the invasion: A tame asteroid would also make one hell of a weapon of mass destruction. Tens to millions of times the blast of a nuke, but no radioactive fallout. From cuts and bruises from the asteroid strike on Chelabinsk to the extinction of the dinosaurs, mother nature has given us a blueprint for a terrifying weapon.

As an invading alien, I've got technology centuries ahead of present day humanity's to work with. I also have lots of time, and lots of space rocks: The main asteroid belt has over 200 asteroids larger than 100 kilometres and 750,000 asteroids larger than 1 km. 

The Kuiper belt beyond Neptune contains thousands of icy objects over 100km wide, and trillions of smaller comets. With enough time I can weaponise almost all of them. 

As we looked at in 'What if the biggest NEO was going to hit us?' the human race would be stretched to knock aside any asteroid much bigger than 10 km on short notice. Once I've made a few thousand of them dance the fandango in Hubble's field of view** - or flattened a couple of cities with some smaller asteroids - humanity would face a stark choice: Surrender to me unconditionally, or face the end of life on Earth. Because I could literally pummel this planet to the point of sterilisation.

Comet 67-P would nicely

Brute power isn't the only advantage over an invasion with ships and troops: Humans do in fact have some weapons for in-space combat. But an invader who can throw half the asteroid belt need never come within range of a human jammer, kinetic impactor, or ASAT missile***. 

So to invade another planet - at least at Earth's sort of tech level - threatening to drop rocks seems far easier than  invading Star Wars style.

That's not the scary bit. The scary bit is how close humans are to having this ability ourselves. A weapon with many times a nuke's punch but  none of the radiation? We might see humans threatening each other with them long before we meet any aliens smart enough to do it.

* Because what could wrong with that?

** Not literally, because I'm an alien with no idea what the fandango is.

***By the way, I apologise to the scriptwriters of the first Independence Day movie: Their one original bit of plot - attacking the alien mothership with a computer virus - wouldn’t work: They’d never need to get that close to Earth. Not that that spoils the film in the slightest – I have made gifs of Will Smith punching that alien on an endless loop, because I have a simple and violent sense of humour, as would, I suspect, the real aliens bombing us with remote controlled asteroid would. 

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